English: The light path through a typical microscope set up for Köhler illumination. The four diagrams illustrate the light path from, and the resulting conjugate image planes, of various components within the optical setup. The top light path shows the conjugate image planes of the filament of the halogen bulb which illuminates the sample, the second shows the conjugate image planes of the field diaphragm, the third shows the conjugate image planes of the aperture diaphragm and the bottom shows the conjugate image planes of the specimen. For correct Köhler illumination the image of the bulb filament must fully fill the aperture diaphragm and the field diaphragm must share the same conjugate image planes as the sample. This results in extremely even illumination of the sample due to perfect defocus of the bulb filament in the specimen image planes so generates a clear image of the sample.